Research in Focus

FBH lasers for investigations of climate-relevant atmospheric methane

The FBH has developed tailored laser diode benches based on FBH’s proprietary QCW laser bars for pumping an ultra-precise LiDAR system to be launched into orbit in the time frame 2022/2023, as part of the French-German space cooperation, MERLIN. The Fraunhofer ILT in Aachen, Germany is responsible for the Laser-Opto-Mechanical-Assembly of the laser transmitter that generates high-power single-frequency pulses which are spectrally matched to a methane absorption line. The MERLIN satellite mission will advance climate and environmental investigations by measuring methane concentrations in the Earth’s atmosphere with optical pulses, characterizing natural and anthropogenic sources of the gas. Methane is one of the most detrimental greenhouse gases.

FBH demonstrates fully digital transmitter chain

Digital power amplifier module

In common transmitter systems the baseband signal is usually generated in the digital domain, utilizing powerful but yet low-cost digital signal processors. However, upconversion and amplification are carried out in the analog domain. Recently, a fully digital transmitter chain was demonstrated at FBH, combining modulator and amplifier into one coherent system. This achievement is an important step towards the usability of such systems in real-world applications. Using a realistic test signal, the overall system performance could meet high standards regarding signal quality and linearity.

The system is designed as a drop-in replacement for present analog transmitter chains so that existing applications can be easily converted without requiring changes on the receiving side. The switch-mode amplifier MMIC fabricated uses FBH’s GaN-HEMT process. While a traditional Tx chain has to utilize a power-hungry digital predistortion to correct non-idealities of the amplifiers, FBH’s digital PA modulator already incorporates provisions to eliminate them without any additional building blocks. Correction parameters can be estimated based on the system’s live signal.

For demonstration purposes, up to 20 MHz WCDMA-like signals were used on a 900 MHz carrier. An ACLR of more than 52 dB in the neighboring channels was measured. Competitive efficiency results and linearity behavior were achieved. The system utilizes an easy-to-construct lumped LC filter as a bandpass with very low insertion loss while maintaining state-of-the-art linearity.

High-power, diffraction-limited, and tunable narrow linewidth diode laser sources emitting in the near infrared are needed in applications like absorption spectroscopy, bio-medical imaging, and particularly for frequency conversion.

The DBR gratings for the dual wavelength laser are designed for 973.67 nm and 975.91 nm, respectively. The emission width is smaller than 17 pm. Wavelength tuning of this device – 7.5 nm from each arm – is obtained by using an electrically controlled micro-heater, implemented on top of the grating sections of the MO. Together with the spectral distance of 2.23 nm between the two arms, wavelength tuning adds up to 9.7 nm. Results indicate that a total combined tunability of about 7.5 nm x NArms can be achieved. The PA ensures the amplification from both branches, with output powers of about 5.5 W. The emitted light features a propagation factor of M2 = 2.2 along the slow axis, and a power content of 72% within the central lobe (4 W diffraction-limited).